1. Field of the Invention
The invention relates generally to the field of photographic lenses having a variable focal length, commonly referred to as zoom lenses. More particularly, the invention relates to a zoom lens which is changed in focal length by moving first and second lens groups along an optical axis and is focused by moving the first lens group along the optical axis.
2. Description of the Prior Art
The basis of all zoom lenses is the fact that in a lens system consisting of positive and negative lens elements a change in the separation of the lens elements changes the focal length. This, however, alters the lens to image distance for maximum sharpness. Thus, a change in the focal length requires that the zoom lens be refocused.
Typically in many zoom lenses, the focal length is changed by moving first and second lens groups in different amounts along an optical axis and is focused by moving the first lens group relative to the second lens group along the optical axis. Examples of such an arrangement are shown in U.S. Pat. Nos. 4,275,952 granted June 30, 1981 and in 4,472,033, granted Sept. 18, 1984. In the 4,275,952, an operating ring is turned to move a first lens group along the optical axis for focusing by virtue of a helicoidal screw engagement between a lens holding frame and a lens moving frame. On the other hand, the operating ring is moved parallel to the optical axis to move the first and second lens groups in different amounts along the optical axis to change the focal length by means of the helicoid engagement and a camming device. However, this arrangement is mechanically complex and therefore expensive to manufacture.